General form

Mozilla/5.0 is the general token that says the browser is Mozilla compatible, and is common to almost every browser today.

platform describes the native platform the browser is running on (e.g. Windows, Mac, Linux or Android), and whether or not it's a mobile phone. Firefox OS phones simply say "Mobile"; the web is the platform. Note that platform can consist of multiple "; "-separated tokens. See below for further details and examples.

rv:geckoversion indicates the release version of Gecko (such as "17.0"). In recent browsers, geckoversion is the same as firefoxversion.

Gecko/geckotrail indicates that the browser is based on Gecko.

On Desktop, geckotrail is the fixed string "20100101"

Firefox/firefoxversion indicates the browser is Firefox, and provides the version (such as "17.0").

from Firefox 10 on mobile, geckotrail is the same as firefoxversion

The recommended way of sniffing for Gecko-based browsers (if you have to sniff for the browser engine instead of using feature detection) is by the presence of the "Gecko" and "rv:" strings, since some other browsers include a "like Gecko" token.

For other products based on Gecko, the string can take one of two forms, where the tokens have the same meaning except those noted below:

appname/appversion indicates the application name and version. For instance, this could be "Camino/2.1.1", or "SeaMonkey/2.7.1".

Firefox/firefoxversion is an optional compatibility token that some Gecko-based browsers may choose to incorporate, to achieve maximum compatibility with websites that expect Firefox. firefoxversion will generally represent the equivalent Firefox release corresponding to the given Gecko version. Some Gecko-based browsers may not opt into using this token; for this reason, sniffers should be looking for Gecko — not Firefox! Whether this token appears is controlled by the "general.useragent.compatMode.firefox" boolean pref.

Mobile and Tablet indicators

Requires Gecko 11.0(Firefox 11.0 / Thunderbird 11.0 / SeaMonkey 2.8)

The platform part of the UA string indicates if Firefox is running on a phone-sized or tablet device. When Firefox runs on a device that has the phone form factor, there is a Mobile; token in the platform part of the UA string. When Firefox runs on a tablet device, there is a Tablet; token in the platform part of the UA string instead. For example:

The version numbers are not relevant. Avoid inferring materials based on these.

The preferred way to target content to a device form factor is to use CSS Media Queries. However, if you use UA sniffing to target content to a device form factor, please look for Mobi (to include Opera Mobile, which uses "Mobi") for the phone form factor and do not assume any correlation between "Android" and the device form factor. This way, your code will work if/when Firefox ships on other phone/tablet operating systems or Android is used for laptops. Also, please use touch detection to find touch devices rather than looking for "Mobi" or "Tablet", since there may be touch devices which are not tablets.

Firefox OS devices identify themselves without any operating system indication; for example: "Mozilla/5.0 (Mobile; rv:15.0) Gecko/15.0 Firefox/15.0". The web is the platform.

Windows

Windows user agents have the following variations, where x.y is the Windows NT version (for instance, Windows NT 6.1).

Android (version 40 and below)

Form factor

Gecko user agent string

Phone

Mozilla/5.0 (Android; Mobile; rv:40.0) Gecko/40.0 Firefox/40.0

Tablet

Mozilla/5.0 (Android; Tablet; rv:40.0) Gecko/40.0 Firefox/40.0

Android (version 41 and above)

Beginning in version 41, Firefox for Android will contain the Android version as part of the platform token. For increased interoperability, if the browser is running on a version below 4 it will report 4.4. Android versions 4 and above will report the version accurately. Note that the same Gecko—with the same capabilities—is shipped to all version of Android.

Form factor

Gecko user agent string

Phone

Mozilla/5.0 (Android 4.4; Mobile; rv:41.0) Gecko/41.0 Firefox/41.0

Tablet

Mozilla/5.0 (Android 4.4; Tablet; rv:41.0) Gecko/41.0 Firefox/41.0

Firefox OS

Form factor

Gecko user agent string

Phone

Mozilla/5.0 (Mobile; rv:26.0) Gecko/26.0 Firefox/26.0

Tablet

Mozilla/5.0 (Tablet; rv:26.0) Gecko/26.0 Firefox/26.0

TV

Mozilla/5.0 (TV; rv:44.0) Gecko/44.0 Firefox/44.0

Device-specific

Mozilla/5.0 (Mobile; nnnn; rv:26.0) Gecko/26.0 Firefox/26.0

Device-specific user agent strings

Although it is strongly discouraged by Mozilla, some handset manufacturers unfortunately include a token in their device's UA string that represents their device id. If this is the case, the Firefox OS UA string will look like the device-specific string in the table above, where nnnn; is the manufacturer's code for the device (see Guidelines). Some of them we have noticed are of the form "NexusOne;", "ZTEOpen;", or "Open C;" (note that putting space is also discouraged). We provide this information to assist with your UA detection logic, but Mozilla discourages the detection of a device id in UA strings.

Here is a JavaScript regular expression that will detect all mobile devices, including devices with a device id in their UA string:

/mobi/i

The i makes it case-insensitive, and mobi matches all mobile browsers.

Firefox OS version number

While the version number for Firefox OS is not included in the UA string, it is possible to infer version information from the Gecko version number present in the UA string.

Firefox OS version number

Gecko version number

1.0.1

18.0

1.1

18.1

1.2

26.0

1.3

28.0

1.4

30.0

2.0

32.0

2.1

34.0

2.2

37

2.5

44

It's easy to find the correspondences by looking at the Mercurial repository names: repositories starting by mozilla-b2g are the release repositories for Firefox OS, and have both Firefox OS and Gecko versions in their names.

Firefox OS has a four-digit version number: X.X.X.Y. The first two digits are owned by the Mozilla product team and denote versions with new features (eg: v1.1, 1.2, etc). The third digit is incremented with regular version tags (about every 6 weeks) for security updates, and the fourth is owned by the OEM.

Implementation notes for applications, vendors, and extensions

Prior to Firefox 4 and Gecko 2.0, it was possible for extensions to add user agent parts through the general.useragent.extra.identifier preferences, (see the obsolete User Agent Strings Reference). But that has not been possible since bug 581008.

In the past, specific plug-ins, add-ons or extensions added user agent parts to notify sites they were installed. The recommended way to do this, if it's absolutely necessary (remember that it slows down every request) is to set a custom HTTP header.